96 LETTERS ON NATURAL MAGIC. 



is as inexpressible in language as it is inconceivable in 

 thought. 



While the human eye has been admired by ordinary 

 observers for the beauty of its form, tho power of its 

 movements, and the variety of its expression, it has excited 

 the wonder of philosophers by the exquisite mechanism of 

 its interior, and its singular adaptation to the variety of 

 purposes which it has to serve. The eyeball is nearly 

 globular, and is about an inch in diameter. It is formed 

 externally by a tough opaque membrane called the sclerotic 

 coat, which forms the white of the eye, with the exception 

 of a small circular portion in front called the cornea. 

 This portion is perfectly transparent, and so tough in its 

 nature as to afford a powerful resistance to external 

 injury. Immediately within the cornea, and in contact 

 with it, is the aqueous humour, a clear fluid, which 

 occupies only a small part of the front of the eye. Within 

 this humour is the iris, a circular membrane with a hole 

 in its centre called the pupil. The colour of the eye 

 resides in this membrane, which has the curious property 

 of contracting and expanding so as to diminish or enlarge 

 the pupil, an effect which human ingenuity has not 

 been able even to imitate. Behind the iris is suspended 

 the crystalline lens in a fine transparent capsule or bag of 

 the same form with itself. It is then succeeded by the 

 vitreous humour, which resembles the transparent white of 

 an egg, and fills up the rest of the eye. Behind the 

 vitreous humour, there is spread out on the inside of the 

 eyeball a fine delicate membrane, called the retina, which 

 is an expansion of the optic nerve, entering the back of the 

 eye, and communicating with the brain. 



A perspective view and horizontal section of the left 

 eye, shown in the annexed figure, will convey a popular 

 idea of its structure. It is, as it were, a small camera 

 obscura, by means of which the pictures of external 



